LOS ANGELES, AUG. 27 -- The Reagan administration is now operating on the "working assumption" that a U.S.-Soviet summit will be held in the United States late in November, White House sources said today.

They said that this anticipated third meeting between President Reagan and Soviet leader Mikhail Gorbachev would focus on the signing of a treaty removing medium-range and short-range nuclear missiles from Europe and Asia but would also deal with regional issues.

Soviet agreement to the treaty is considered likely, but not certain, in the wake of a recent change in the U.S. position on verification and West German willingness to scrap 72 aging Pershing IA missiles that the Soviets considered an obstacle to agreement.

In a speech here Wednesday reviewing U.S.-Soviet relations, the president expressed optimism about approval of the treaty and said that the new Soviet leadership "appears more willing to address the problems that have divided East and West for so long and to seek agreements based on mutual benefit."

But White House officials nonetheless hedged their summit predictions. They emphasized that the Soviets had not yet proposed or agreed to a new summit, although the matter has been discussed informally through diplomatic channels.

The issue is expected to be resolved when Secretary of State George P. Shultz and Soviet Foreign Minister Eduard Shevardnadze hold three days of talks in Washington, beginning Sept. 15. If a summit is agreed to, it could be announced then or the following week when Reagan addresses the U.N. General Assembly, the sources said.

Administration reluctance to discuss the prospective summit publicly reflects a number of considerations, said the White House officials. One of these is a political worry that conservatives who are skeptical about the treaty are likely to become even more critical if Reagan appears overeager for a summit.

Another is persistent tension between the White House and the State Department and a concern that Shultz is especially sensitive in the wake of the Iran-contra affair of any moves that might be seen as upstaging him.

But the most fundamental reason for caution, in the words of one senior official, is the desire to avoid a repetition of the 1986 situation in Reykjavik, Iceland, a snap summit initiated by Gorbachev that Reagan agreed to. At that meeting the two leaders held far-reaching discussions on elimination of strategic nuclear missiles but failed to reach agreement after Reagan refused to yield on deployment of a missile defense system.

Officially, the administration view is that Reykjavik was a success. Privately, the view is closer to the disappointment expressed by Shultz immediately after the Iceland meeting at the negative outcome of what he then referred to as a "high-stakes poker game."

"We don't want another Reykjavik," an official said this week.

The president, Shultz and national security adviser Frank C. Carlucci conferred here today on the arms control situation before meeting with leaders of the Nicaraguan contras.

White House spokesman Marlin Fitzwater denied that the administration was disappointed with the low-key Soviet reaction to West German Chancellor Helmut Kohl's proposal to scrap the Pershing missiles.